(1) Field of the Invention
The present invention pertains to transmission of messages between two systems controlled by central processing units and more particularly to a connection of two central processing units to a common teletypewriter providing for interlock control of input and output messages sent between the two central processing units and the common teletypewriter.
(2) Description of the Prior Art
With the advent of electronic telephone switching systems, it is necessary that maintenance of the sophisticated equipment located in such systems be accomplished. The maintenance of such equipment as computer central processing units, electronic line and trunk circuitry and input/output equipment associated with a computer require well trained and knowledgeable craftsmen in order to maintain the proper operation of the switching system.
Typically, each electronic switching center would require such well-trained personnel to be present at the particular site should problems arise. The cost of providing such persons at each site is high. In addition, such skilled personnel would have to be provided on an around the clock basis, since telephone service must be provided 24-hours a day.
Therefore, in an effort to economize on the technically skilled manpower required, such telephone manufacturers as GTE Automatic Electric have developed systems with the ability to monitor from a remote location a number of electronic switching centers. One such system manufactured by GTE Automatic Electric is the Remote Monitor and Control System (RMCS). The RMCS system permits remote monitoring and maintenance of up to 16 electronic telephone switching centers. Each electronic telephone switching center has an input/output terminal device such as a teletypewriter. This terminal device is used for input and output of messages concerning the configuration and maintenance activity of a particular switching center. One such electronic telephone switching center (EAX) is manufactured by GTE Automatic Electric and is described, showing the organization and input/output structure, in U.S. Pat. Nos. 3,820,084 issued on June 25, 1974 to L. V. Jones et al and 3,831,151 issued to L. V. Jones et al on Aug. 20, 1974.
Prior to remote monitoring systems such as RMCS, a simple solution to remote monitoring of the maintenance terminal device of each electronic switching center was accomplished in the following way. A second terminal device was connected in parallel with the existing maintenance terminal device via a dedicated telephone line. Originally this parallel connected remote terminal device performed in only a monitor capacity. That is, the terminal would print all the messages as printed by the local maintenance terminal device and would not be capable of providing input from the remotely located terminal device.
The next feature that was provided was that of giving the ability of the remotely located maintenance terminal device to input messages to the electronic switching center. The problem thereby resulting was that of overlaping input requests from the local and the remote maintenance terminal device. Since the craftsman at the local maintenance terminal would not be aware of what functions the craftsman located at the remote maintenance terminal was performing, nearly simultaneous input requests could easily be generated by the local and remote terminal devices. The electronic switching center could not determine whether the local or remote maintenance device had made the input request, thereby receiving errors in the transmission of the message.
Since the RMCS system monitors up to 16 electronic switching centers, it is of critical importance that input requests to each switching center are not repeated because of errors in reception by the electronic switching center due to the above mentioned problem.
U.S. Pat. Nos. 3,506,794, issued on Apr. 14, 1970, to W. Chulak; 3,892,928, issued on July 1, 1975, to R. Casterline et al; 3,958,111, issued on May 18, 1976, to J. Hackett; and 3,980,839, issued on Sept. 14, 1976, to A. Hutcheson, teach the use of diagnostic monitoring apparatus.
Therefore, it is the object of the present invention to provide a shared maintenance terminal control system for use in message transmission between a data processing center and an electronic telephone switching center including the terminal device, to prevent reception errors during message transmission between the data processing and telephone switching centers by means of interlock control.
It is also a feature of the present invention to provide the capability of selective message routing between the switching center, data processing center and terminal device.